Plastic very fast and convenient - flexible to install in awkward shaped spaces, and you don't need to go in with the soldering iron, so safer, and much less faff.
Also easier to disconnect/uninstall, when you want to change things, or have messed up!
Like previous posters, I always use copper where visible or near to heat sources.
Remember to use lubricant gel (!)
Always use a quality brand (I use Hep2o or Speedfit) - cheap ones do not age well - especially when subject to repeated/prolonged cycles of high temperatures and cooling.
Never put your finger inside the fitting (or let kids) - there are metal teeth in there designed to grip the end of the pipe!
I must admit, I still like the permanency of soldered copper, when the access is good and time is not pressing.